PROJECT SUMMARY/ABSTRACT Staphylococcus aureus is the leading cause of community and nosocomially-acquired infectious and toxin- mediated syndromes, some life-threatening that affect patients of all ages. As a species, S. aureus has become resistant to antibiotics of all classes that have been designed for use in the clinical arena. Disease caused by isolates resistant to methicillin, so-called MRSA isolates, has become epidemic in the US in the last decade. The high prevalence of these isolates necessitates a need for new antimicrobials to treat MRSA isolates that are resistant to all available ss-lactam antibiotics including penicillins and cephalosporins. Moreover, the effectiveness of the glycopeptide antibiotic vancomycin, long regarded as the antibiotic of last resort for MRSA infections, has begun to decrease due to globally decreasing sensitivity among S. aureus and the development of hVISA, VISA and VRSA isolates. Our focus has been on a two-component signal transduction operon that senses cell wall stress and is activated by a variety of cell wall-active antimicrobials. Once activated, this system affects the transcription of a variety of genes, upregulating some and down- regulating others. Inactivation of this operon, called VraSR, produces a methicillin-susceptible phenotype even in strains that have mecA, the gene conferring the MRSA phenotype. The mechanism by which the resistant phenotype is ablated in these mutants is not clear. We plan to evaluate the consequences of further subjecting this cell wall stress operon to genetic manipulation to understand its role in effecting the MRSA resistance phenotype. These studies should provide further insight into the mechanisms by which MRSA isolates elude antimicrobial therapy and form the basis for the search for new targets for antimicrobial therapy.